Beginner: How to make a mirrored "recesses'' in a shape of a spheres into an object mesh?
The following method works better when the faces are evenly spaced quads


Add a sphere
Go to edit mode
Press "A" to select all faces
Press "I" once or twice
Scale the individual faces down
Press "E" to extrude all faces
Press "S" to scale the faces to the center
After scaling it towards the sphere center apply
Add a Subdivision Surface Modifier 


I just found a tutorial that has another nice method: https://www.youtube.com/watch?v=nJXsgan3Juc